1. Field of the Invention
The present invention relates to a driving system for a reluctance type motor, particularly to a driving system capable of removing torque ripple components included in output torque of the reluctance type motor preferable for use as driving sources for various apparatus.
2. Description of the Prior Art
There has been known a reluctance type motor which comprises a stator having a plurality of magnetic poles and a rotor having a plurality of salient-poles and causes the rotor to rotate by magnetic attraction interacting between sequentially excited magnetic poles of said stator and corresponding salient-poles of said rotor facing the excited magnetic poles. The reluctance type motor has advantages such as remarkably large output torque, good efficiency, and inexpensive cost, but to the contrary, has a disadvantage such as limited application due to its large torque ripples. That is, in the reluctance type motor, remarkably large torque is generated shortly after salient-poles begin to face the magnetic poles. On the other hand, the torque becomes small just before the salient-poles completely face the magnetic poles.
Accordingly, output torque is generated with ripple components. And further, as exciting current increases, a curve representing the relationship between torque and rotor rotational angle increases its height as well as generating a point of its peak value advanced earlier. Therefore, conventional reluctance type motors cannot be used as driving sources for precise servo-motors etc. which need to satisfy the requirement that the torque ripple must be limited to less than a few % of the output torque under the condition that the reluctance type motor is driven with less than 30% of the rated output torque.
For removing torque ripples in a DC-motor with a magnetic rotor, there is known a technology that changes the exciting current in accordance with the rotational angle of the rotor (for example, refer to Japanese Unexamined Patent Application No. SHO 55-44555). According to this proposed technology, a curve representing the relationship between the torque and the rotor rotational angle for the DC-motor becomes flat and therefore torque ripple can be removed. That is, the torque-rotational angle curve merely varies in a similar way in response to the increase and decrease of the exciting current in a DC-motor. However, in reluctance type motors, this torque-rotational angle curve varies in a complicated way in response to the increase and decrease of the exciting current. Therefore, it is difficult to obtain a flat torque characteristic by removing torque ripples